In order to reveal the preliminary inhibition mechanisms of aquatic plants bio-oils on Skeletonema costatum, effects of Arundo donax L. 300 degees C, Ph. australis Trin. 400 degrees C and Typha orientalis Pres1 400 degrees C bio-oils on the concentration change of malondialdehyde (MDA) and the activity of antioxidant enzymes system (SOD, POD and CAT) were evaluated. The results showed that the higher Ihe Bio-oil concentrations, the higher the MDA contents in Skeletonema costatum was, and when the Bio-oil concentration was 10 mg.L-1 the MDA concentration increased with the reaction time. Superoxide dismutase (SOD) activity also increased with the increase of bio-oil concentration. For Arundo donax L 300 degrees C and Typha orientalis Presl 400 degrees C bio-oil, when the reaction time was longer, the S0D activity of Skeletonema costatum first increased and then decreased, and in both cases the maximum SOD activity was measured at 24 h. reaching 93.6 U (10(7) cells)-1 and 8.23 U (10(7) cells)-1, respectively. For Ph. australis Trin 400 degrees C bio-oil, the SOD activity kept increasing within 72 h. The peroxidase ( POD) activity of Skeletonema costatum also increased with the increase of bio-il concentrations. In the presence of Arundo donax L. 300 degrees C and Ph. australis Trin 400 degrees C bio-oil, the POD activity of Skeletonma, costatum first increased and then decreased, while with Typha orientalis Presl 400 degrees C bio-oil the POD activity increased with fluctuations. For all the three bio-oils, the catalase (CAT) activities increased first and then decreased when the reaction time was prolonged, and the higher the bio-oils concentration, the greater the CAT activity was. Pyrolysis bio-oils enhance the activity of antioxidant enzymes, leading to intracellular oxidative stress in the algae, which seems to be the main inhibitory mechanism for algae